Indian leader Narendra Modi said on Thursday (Apr 7) a fatal flyover collapse in Kolkata was "an act of fraud" and accused West Bengal Chief Minister Mamata Banerjee of corruption at a state election rally.

Modi, campaigning for his Hindu nationalist Bharatiya Janata Party (BJP), said the collapse was the result of corrupt practices under Banerjee's ruling Trinamool Congress state government.

"A huge bridge collapsed and what did these people say - an act of God. Didi (Banerjee), this is not an act of God but an act of fraud," Modi said at the rally in Birpara, a town in the state's far northeast.

Twenty-six people were killed when a portion of a flyover that was under construction collapsed onto a busy street crushing scores of pedestrians and cars on Mar 31.

A top official from the construction company IVRCL that was building the bridge drew ire for saying the collapse was "an act of God", although Banerjee herself is not known to have said this.

Nine officials of the firm have been arrested on charges including murder as investigators probe negligence in the disaster, which came on the brink of a month-long state election expected to be closely fought.

Modi said it was a divine message to voters in the state to reject the chief minister's party in the polls over what he alleged was rampant corruption.

"This business of money and death led to the collapse of the Vivekananda bridge," Modi said. "In a sense it is an act of God that it fell on election eve to expose your (Banerjee's) misgovernance," he said.

The BJP, which has failed to gain ground in West Bengal and won just one seat in the previous election, is trying to put the ruling party on the back foot over the incident.

Derek O'Brien, a lawmaker and chief national spokesman for the Trinamool Congress, accused Modi in a social media post of "playing cheap politics over a tragedy".

"These are not words befitting of the Prime Minister of India. They bring his high office to disrepute ... The Trinamool Congress is far from being a corrupt party," he wrote on Facebook.

Banerjee's party is facing a two-way challenge in the elections after previously ending the 34-year-rule of Leftist parties, which have entered into an alliance with the Indian National Congress.

West Bengal is one of five Indian states holding elections this month and next and the results will be announced on May 19.